Image forming apparatus for forming test pattern on sheet using coloring material of plurality of colors

ABSTRACT

The image forming apparatus includes: a measuring unit configured to measure reflected light from a test sheet, the measuring unit including a first sensor and a second sensor, a generation unit configured to generate information regarding a color of a plurality of measurement images based on a measurement result by the measuring unit; and a detection unit configured to detect a sensor that has output an abnormal measurement result in the measuring unit, based on the measurement result. The detection unit is further configured to detect a sensor that has output the abnormal measurement result, based on measurement results obtained by the measuring unit regarding reflected light from a predetermined region of the test sheet. The predetermined region corresponds to a region on which the plurality of measurement images are not formed.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to stabilization control for images formedby image forming apparatuses.

Description of the Related Art

Color reproducibility is important for image forming apparatuses. Inorder to improve color reproducibility, US 2006/198648 discloses aconfiguration in which the density of an image formed on a sheet isdetected by a reader unit in an image forming apparatus, which is acopying machine, and image forming conditions are controlled. US2006/198648 discloses a configuration in which an RGB sensor is providedin the image forming apparatus, and an image formed on a sheet is readby the RGB sensor to control the image forming conditions. Furthermore,US 2004/042807 discloses a configuration in which image formingconditions are controlled using a spectral reflectance sensor, which hasa wider reading area and higher accuracy than those of an RGB sensor.The configurations described in US 2006/198648 and US 2004/042807 arefor detecting an image formed on a sheet while conveying the sheet inthe image forming apparatus. However, if a sheet that is being conveyedflaps, reflected light from the sheet varies and the adjustment accuracyof the image forming conditions is affected. Japanese Patent Laid-OpenNo. 11-11718 discloses an image forming apparatus that has a detectionunit for detecting flapping of a sheet.

With the configuration described in Japanese Patent Laid-Open No.11-11718, a space for providing the detection unit for detectingflapping of a sheet is required, and moreover, the provision of thisdetection unit increases cost. Furthermore, when performing colorimetricmeasurement on an image on a sheet that is being conveyed, not onlyflapping of the sheet but also unevenness in the sheet conveyance speedaffect the measurement accuracy. For example, if, at a colorimetricmeasurement timing for an image, colorimetric measurement is performedon a part of the previous image or the next image due to the unevennessin the conveyance speed, a difference in tone called a tone jump occursunder an image forming condition that is based on the result of thiscolorimetric measurement. Moreover, in the case of performingcolorimetric measurement on a large number of colors in order to createa multi-color lookup table (LUT), colorimetric measurement needs to beperformed on an image with more than 200 colors. The number of sensorsneeds to be increased to perform colorimetric measurement on an imagewith such a large number of colors, but in this case, a tone jump occursdue to a difference in reading accuracy among the sensors.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, an image formingapparatus includes: an image forming unit configured to form an image ona sheet; a conveyance unit configured to convey the sheet along aconveyance path; a measuring unit configured to measure reflected lightfrom a test sheet conveyed by the conveyance unit, the measuring unitincluding a first sensor and a second sensor, the second sensor beingprovided at a position different from a position of the first sensor ina direction perpendicular to a conveyance direction in which the sheetis conveyed by the conveyance unit; a controller configured to controlthe image forming unit to form, on the sheet, a plurality of measurementimages for creating the test sheet, control the conveyance unit toconvey the test sheet, and control the measuring unit to measure thereflected light from the test sheet; a generation unit configured togenerate information regarding a color of the plurality of measurementimages based on a measurement result obtained by the measuring unit; anda detection unit configured to detect a sensor that has output anabnormal measurement result in the measuring unit, based on measurementresults obtained by the measuring unit regarding reflected light from apredetermined region of the test sheet. The predetermined regioncorresponds to a region on which the plurality of measurement images arenot formed.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of an image formingapparatus according to an embodiment.

FIG. 2 is a diagram showing a control configuration of the image formingapparatus according to an embodiment.

FIG. 3 is a diagram showing an operation panel according to anembodiment.

FIG. 4 is a diagram showing a configuration of a color detection sensoraccording to an embodiment.

FIGS. 5A and 5B are diagrams showing a configuration at the periphery ofthe color detection sensor at the time of white reference platemeasurement according to an embodiment.

FIGS. 6A and 6B are diagrams showing a configuration at the periphery ofthe color detection sensor at the time of colorimetric measurementaccording to an embodiment.

FIGS. 7A to 7C are diagrams showing a test pattern according to anembodiment.

FIG. 8 is a flowchart of automatic hue correction according to anembodiment.

FIGS. 9A to 9C are diagrams illustrating a coloring material used inpatches of a test pattern according to an embodiment.

FIG. 10 is a diagram showing values to be calculated and items to becorrected in respective corrections.

FIG. 11 is a diagram showing an allowable range for color differencesaccording to an embodiment.

FIG. 12 is a diagram showing content to be displayed on the operationpanel according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed with reference to the drawings. Note that the followingembodiments are examples, and are not intended to limit the presentinvention to the content of the embodiments. Constituent elements thatare not necessary for the description of the embodiments are omitted inthe diagrams used below.

First Embodiment

This embodiment will be described below regarding an electrophotographicimage forming apparatus. However, the present invention is alsoapplicable to inkjet and sublimation image forming apparatuses. FIG. 1is a cross-sectional diagram showing a structure of an image formingapparatus 100 according to this embodiment. An engine control CPU 102and a printer controller 300 that performs image processing and thelike, which are shown in FIG. 2, are housed in a control board housingunit 104 of the image forming apparatus 100. Note that these controlprocesses (e.g., paper feed processing) related to image formation. Fourstations 120, 121, 122, and 123 respectively form yellow, cyan, magenta,and black toner images on an intermediate transfer member 106. The fourstations 120, 121, 122, and 123 have the same configuration except forthe toner colors that the respective stations use. A photosensitivemember 105 is a kind of image carrier, and is rotationally driven whenforming an image. A charging unit 111 charges a surface of thephotosensitive member 105 at a uniform potential. An exposure unit 107scans and exposes the charged photosensitive member 105 using light thatis output by a light source 108 and corresponds to an image to beformed, and forms a latent image on the photosensitive member 105. Adevelopment unit 112 develops the latent image using toner, which is acoloring material, to form a toner image. This toner image is primarilytransferred onto the intermediate transfer member 106. A transfer roller114 secondarily transfers the toner image formed on the intermediatetransfer member 106 onto a sheet 110 that has been conveyed from a tray113. In this manner, the four stations, the intermediate transfer member106, and the transfer roller 114 function as a formation unit that formsan image on the sheet 110 using a plurality of coloring materials.

A fixing processing mechanism in the image forming apparatus 100according to this embodiment has a first fixing unit 150 and a secondfixing unit 160 that heat and press the toner image, which has beentransferred onto the sheet 110, to fix the toner image onto the sheet110. The first fixing unit 150 includes a fixing roller 151 for heatingthe sheet 110, a pressure belt 152 for pressing the sheet 110 againstthe fixing roller 151, and a first post-fixing sensor 153 that detectscompletion of the fixing. The second fixing unit 160 is arranged on thedownstream side of the first fixing unit 150 in the conveyance directionof the sheet 110. The second fixing unit 160 adds gloss to the tonerimage on the sheet 110 fixed by the first fixing unit 150, and ensuresthe fixability. Similar to the first fixing unit 150, the second fixingunit 160 also has a fixing roller 161, a pressure roller 162, and asecond post-fixing sensor 163. Depending on the type of sheet 110, thesheet does not need to pass through the second fixing unit 160. In thiscase, in order to reduce energy consumption, the sheet 110 passesthrough a conveyance path 130 without passing through the second fixingunit 160. A conveyance path switching flapper 131 switches betweenguiding the sheet 110 to the conveyance path 130 and guiding the sheet110 to the second fixing unit 160.

A conveyance path switching flapper 132 is a guiding member that guidesthe sheet 110 to a conveyance path 135 leading to an inversion unit 136or guides the sheet 110 to a discharge path 139 leading to the outside.An inversion sensor 137 is provided in the conveyance path 135. Aleading end of the sheet 110 passes through the inversion sensor 137,and the sheet 110 is conveyed to the inversion unit 136. Upon theinversion sensor 137 detecting a trailing end of the sheet 110, theconveyance direction of the sheet 110 is switched. A conveyance pathswitching flapper 133 is a guiding member that guides the sheet 110 to aconveyance path 138 for two-sided image formation, or guides the sheet110 to the conveyance path 135. A conveyance path switching flapper 134is a guiding member that guides the sheet 110 to the discharge path 139leading to the outside. Note that a large number of conveyance rollersare provided in the conveyance paths including the conveyance path 135and the discharge path 139.

A color detection sensor 200 that detects a test pattern formed on thesheet 110 is arranged downstream of the second fixing unit 160 in theconveyance direction of the sheet 110. In this embodiment, the colordetection sensor 200 is a spectral reflectance sensor that irradiatesthe test pattern with light and detects a spectral reflectance of thereflected light. Note that a white reference plate 250 is provided onthe side opposite to the color detection sensor 200 relative to theconveyance path 135. The color detection sensor 200 functions as acolorimetric measurement unit that performs colorimetric measurement ona color of the image fixed on the sheet 110, downstream of the firstfixing unit 150 and the second fixing unit 160 in the conveyancedirection of the sheet 110. The color detection sensor 200 may bearranged in the discharge path 139. An operation panel 180 functions asan input/output unit from which a user inputs an instruction to theimage forming apparatus, and that displays a state of the image formingapparatus to the user, for example.

FIG. 2 is a diagram showing a control configuration of the image formingapparatus 100 according to the present embodiment. A host computer 301is a computer that transmits a print job to the image forming apparatus100 via a wired or wireless communication line. The printer controller300 operates in conjunction with the engine control CPU 102 to controlthe operation of the image forming apparatus 100. Units that constitutethe printer controller 300 are connected to one another via a bus 319.

A host I/F unit 302 is a communication unit that manages input andoutput to/from the host computer 301. An input/output buffer 303 storesa control code from the host I/F unit 302 and accumulates data fromcommunication units. A printer controller CPU 313 is the main processorthat comprehensively controls overall operation of the image formingapparatus 100. A ROM 304 is a memory that stores a control program andcontrol data of the printer controller CPU 313. Functions achieved bythe printer controller CPU 313 executing this control program include,for example, an image information generation unit 305, an uneven densitycorrection table generation unit 306, a tone correction table generationunit 307, a multi-color table generation unit 308, and the like.

The tone correction table generation unit 307 generates and updates atone correction table (γLUT), which contains correction conditions foradjusting monochromatic tones through automatic tone correction. Themulti-color table generation unit 308 generates and updates amulti-color LUT, which contains correction conditions for colorcorrection, that is, an ICC profile, through automatic hue correction.Furthermore, the uneven density correction table generation unit 306generates and updates a light intensity table through uneven densitycorrection. Note that the uneven density correction is a control forcorrecting the light intensity of the light source 108 in order tocorrect the uneven density in the main scanning direction.

A RAM 309 is a memory that is used as a work area for interpreting thecontrol code and data, performing calculation necessary for printing, orprocessing print data, and a table storing unit 310 is provided therein.The ICC profile generated by the multi-color table generation unit 308,the tone correction table created by the tone correction tablegeneration unit 307, and a light intensity table created by the unevendensity correction generation unit 306 are stored in the table storingunit 310. An image information generation unit 305 generates variousimage objects (a test pattern etc.) in accordance with settinginformation received from the host computer 301. A RIP (Raster ImageProcessor) unit 314 is a processor that expands an image object into abitmap image. A color processing unit 315 performs color conversionprocessing in accordance with the ICC profile, which is a color profilegenerated by the multi-color table generation unit 308. A tonecorrection unit 316 executes monochromatic tone correction using thetone correction table created by the tone correction table generationunit 307. A pseudo halftone processing unit 317 performs pseudo halftoneprocessing, such as dither matrix and an error diffusion method, onimage data. An engine I/F unit 318 is a communication unit thattransfers image data and light intensity information indicated by thelight intensity table to the engine control CPU 102. The engine controlCPU 102 controls the four stations 120, 121, 122, and 123, and the liketo form an image in accordance with the image data.

The operation panel 180 is constituted by a display device and an inputdevice, accepts input of instructions to execute printing and correctionprocessing, and displays information to the user, for example. FIG. 3 isan exemplary screen displayed by the operation panel 180. FIG. 3 shows ascreen that is displayed when the user selects “adjustment/CLN(cleaning)”, which relates to the present invention, and automatic tonecorrection, automatic hue correction, uneven density correction aredisplayed therein. As a result of the user selecting any of thesecorrections, the image forming apparatus 100 executes the selectedcorrection control. A panel I/F unit 311 connects the operation panel180 and the printer controller 300 to each other. Note that thecolorimetric measurement result obtained by the color detection sensor200 is input as measurement data to the printer controller CPU 313. Theprinter controller CPU 313 manages the ICC profile, the tone correctiontable, and the light intensity table that are used when forming an imageand updates them based on the measurement data as necessary, therebyenabling a desired color to be output.

FIG. 4 is a diagram showing a configuration of the color detectionsensor 200. A white LED 201 is a light-emitting device that irradiates atest pattern 220 on the sheet 110 with light. A diffraction grating 202is a spectral component that diffracts light reflected from the testpattern 220 for each wavelength. A line sensor 203 is a light detectiondevice that includes n light-receiving devices for detecting light thatis spectrally resolved for each wavelength by the diffraction grating202. A computing unit 204 performs various kinds of computation usinglight intensity values of respective pixels detected by the light sensor203. Various data used by the computing unit 204 is saved in a memory205. Note that a lens 206 that collects light emitted from the white LED201 onto the test pattern 220 on the sheet 110 and collects lightreflected from the test pattern 220 onto the diffraction grating 202 mayfurther be provided. The color detection sensor 200 measures a color ofthe test pattern on the sheet 110 that is being conveyed by a conveyanceunit. The white reference plate 250 is used for adjusting the lightintensity of the white LED 201, for example. Note that, in the case ofinstalling a plurality of color detection sensors 200 in the imageforming apparatus 100, only one pair of the computing unit 204 and thememory 205 may be provided for the plurality of color detection sensors200. This is because, with this configuration, processing related tomeasured values from the plurality of color detection sensors 200 can beexecuted in an integrated manner, and the load on the printer controller300 can be reduced. Furthermore, since the number of parts can bereduced, the effect of reducing manufacturing costs can also bedemonstrated.

FIGS. 5A, 5B, 6A, and 6B are diagrams illustrating a configuration ofthe color detection sensor 200. Note that FIGS. 5A and 6A are diagramsas viewed in the same direction as in FIG. 1, and FIGS. 5B and 6B arediagrams as viewed in a direction A indicated in FIGS. 5A and 6A. FIGS.5A and 5B show a state in the case of calibrating the color detectionsensor 200, and FIGS. 6A and 6B shows a state of withdrawing the whitereference plate 250 and detecting a test pattern formed on the sheet110. A backup roller 2003 is arranged between a conveyance roller 2002and a conveyance roller 2004 in the sheet conveyance direction, similarto the color detection sensor 200. Note that the backup roller 2003 isprovided on the side opposite to the color detection sensor 200 relativeto the conveyance path 135. The backup roller 2003 abuts against theconveyance path 135 and suppresses flapping of the sheet 110 when thetest pattern is measured. Note that a shutter 2001 is for protecting thewhite reference plate 250 from becoming unclean due to paper powder orthe like. When calibrating the color detection sensor 200, in order tocause the color detection sensor 200 to detect reflected light from thewhite reference plate 250, the printer controller CPU 313 withdraws theshutter 2001 and causes the white reference plate 250 to abut againstthe conveyance path 135, as shown in FIGS. 5A and 5B. On the other hand,for example, in the case of performing colorimetric measurement on thetest pattern, the printer controller CPU 313 withdraws the whitereference plate 250, moves the shutter 2001 to a protection position,and causes the backup roller 2003 to abut against the conveyance path135, as shown in FIGS. 6A and 6B.

The color detection sensor 200 detects 35-band spectral reflectanceinformation bands at intervals of 10 nm with a wavelength of 380 nm to720 nm, for example. In this embodiment, this spectral reflectanceinformation is converted into information shown in FIG. 10 by the colordetection sensor 200 in accordance with the content of correctioncontrol. For example, in the automatic tone correction, the spectralreflectance information is converted into a density value via a status Afilter and a visual filter, and generates and modifies the tonecorrection table based on this density value. In the automatic huecorrection, the spectral reflectance information is converted into CIEL*a*b* through a color matching function, which is a conversion methodin accordance with ISO 13655, and a standard light source D50(hereinafter referred to as SD50), and generates and modifies the ICCprofile, which is a multi-color LUT. Furthermore, in the uneven densitycorrection, the spectral reflectance information is converted into adensity value, and the light intensity table is generated and updated soas to correct the uneven density in the main scanning direction.

Test patterns that are used in the automatic tone correction and theuneven density correction according to this embodiment are monochromaticpatterns of yellow, cyan, magenta, and black, and each color pattern isformed and detected in one sheet. On the other hand, in the automatichue correction, since the multi-color LUT is generated and updated, atest pattern that includes many colors, that is, a test pattern thatincludes multi-color patches formed using multiple types of toner isformed, for example. In this case, even if four color detection sensors200 are arranged in the main scanning direction, a plurality of sheetsare necessary for forming all patches. That is to say, in the automatictone correction and the uneven density correction, only a differencewithin one sheet (hereinafter, “intra-sheet difference”) affects thecorrection control. On the other hand, in the automatic hue correction,a difference between sheets (hereinafter, “inter-sheet difference”) andcharacteristic difference between the color detection sensors 200(hereinafter, “inter-sensor difference”), as well as the intra-sheetdifference, affect the correction control.

FIGS. 7A to 7C show the test patterns used in the automatic huecorrection according to this embodiment. In this example, the testpatterns expand over three sheets, and FIGS. 7A, 7B, and 7C respectivelyshow the test patterns formed on the first, second, and third sheets. Inthis embodiment, four color detection sensors 200 are provided atdifferent positions in the main scanning direction. Accordingly, asshown in FIGS. 7A to 7C, the test patterns formed on the respectivesheets each include four columns that correspond to the four colordetection sensors 200. In FIGS. 7A to 7C, the test pattern in theleft-end column is detected by a first color detection sensor 200(hereinafter referred to as a sensor SC1), and the test pattern in thenext column on the right side thereof is detected by a second colordetection sensor 200 (hereinafter referred to as a sensor SC2).Furthermore, the test pattern in the next column on the right sidethereof is detected by a third color detection sensor 200 (hereinafterreferred to as a sensor SC3), and the test pattern in a right-end columnis detected by a fourth color detection sensor 200 (hereinafter referredto as a sensor SC4).

In the test patterns in FIGS. 7A to 7C, denotes a main-scan triggerpatch, and “P” denotes a pre-scan trigger patch. “C” denotes acolorimetric measurement patch, and a total of 18 colorimetricmeasurement patches are included in each column, excluding the testpattern in the second column from the left on the first sheet shown inFIG. 7A. Note that the test pattern in the second column from the lefton the first sheet includes a total of 17 colorimetric measurementpatches in one column. That is to say, in this embodiment, a total of215 colorimetric measurement patches, that is, colorimetric measurementpatches of 215 colors are formed on the three sheets. Furthermore, inFIGS. 7A to 7C, patches from #1 to #13 are check patches. The checkpatches are each formed by a white solid signal, that is, a signal of acolor signal value of 0%. More specifically, each check patch is not apatch for attaching toner but is a patch that uses a surface of thesheet. Note that in FIGS. 7A to 7C, the check patches #1 and #6 to #13are formed adjacent to the main-scan trigger patches. On the other hand,the check patches #2 to #5 are arranged between colorimetric measurementpatches. For example, in this example, 7 colorimetric measurementpatches are arranged between each of the check patches #2 to #5 and thecorresponding pre-scan trigger patch.

Note that the test patterns in FIGS. 7A to 7C are formed sequentiallyfrom their lower side, and the sheets 110 on which the test patternshave been formed are conveyed to the inversion unit 136 with the lowerside in FIGS. 7A to 7C leading. At this time, the sensors SC1 to SC4perform a pre-scan with detection of the pre-scan trigger patches actingas a trigger. Note that the pre-scan is performed in order to determinecolorimetric measurement conditions such as accumulation time and thenumber of times averaging is to be performed in a main scan to bethereafter performed. The sheets on which the test patterns have beenformed are conveyed to the inversion unit 136 after the pre-scan, andare subsequently conveyed to the conveyance path 135 with the upper sidein FIGS. 7A to 7C leading. At this time, the sensors SC1 to SC4 performcolorimetric measurement on the colorimetric measurement patches and thecheck patches with detection of the main-scan trigger patches acting asa trigger. Note that colorimetric measurement is performed in accordancewith the colorimetric measurement conditions determined through thepre-scan. The sensors SC1 to SC4 obtain color values of the colorimetricmeasurement patches and the check patches from the spectral reflectanceof the colorimetric measurement patches and the check patches.

The printer controller CPU 313 first determines whether or notpredetermined conditions regarding the intra-sheet difference, theinter-sheet difference, and the inter-sensor difference are satisfiedbased on the color values of the check patches #1 to #13. FIG. 11 showsan example of these conditions. In this embodiment, as shown in FIG. 11,the intra-sheet difference is checked based on the color differencebetween the check patches #1 and #3 that are formed in the same columnwithin the same sheet and accordingly subjected to colorimetricmeasurement by the same color detection sensor 200. Specifically, theprinter controller CPU 313 determines a chromaticity 1 of the checkpatch #1 from the spectral reflectance information regarding the checkpatch #1, determines a chromaticity 3 of the check patch #3 from thespectral reflectance information regarding the check patch #3, anddetermines a color difference ΔE based on the chromaticity 1 and thechromaticity 3. If the color difference between the check patches #1 and#3 is smaller than or equal to a threshold value, that is, 1.6 orsmaller in FIG. 11, it is determined that the intra-sheet difference iswithin an allowable range. If a sheet has flapped, the chromaticity 1 ofthe check patch #1 or the chromaticity 3 of the check patch #3 is avalue different from the chromaticity corresponding to paper white, andtherefore, the color difference ΔE is larger than the threshold value.In this embodiment, the inter-sheet difference is checked using thecheck patches formed at the same position on different sheets.Specifically, if the color difference between two check patches detectedby the same color detection sensor 200 among the check patches #6 to #13formed on the second and third sheets is smaller than or equal to athreshold value, that is, 1.6 or smaller in FIG. 11, it is determinedthat the inter-sheet difference is within an allowable range. If anysheet is folded, or the sensor is unclean, the chromaticity of the checkpatches on the different sheets varies. Therefore, folding of a sheet oran unclean sensor can be determined by obtaining the color differencebetween check patches on the sheets. Furthermore, in this embodiment, acolor difference between the average value of color values of the checkpatches #2 to #13 and the average value of the color values of the checkpatches #2 to #13 for each color detection sensor 200 is obtained, andif this color difference is smaller than or equal to a threshold value,that is, 0.8 or smaller in FIG. 11, it is determined that theinter-sensor difference is within an allowable range. In the case wherea detection window of a color detection sensor 200 or the whitereference plate is unclean as well, the color difference between theaverage values of the check patches on each sheet differs. Therefore,the average value of the chromaticity of the check patches is obtainedfor each sheet, and if the color difference for each sheet is largerthan a threshold value, an unclean the detection window and whitereference plate can be determined. Note that the check patch #1 on whichthe sensor SC2 performs colorimetric measurement is excluded from theaverage value processing because the check patch corresponding to thecheck patch #1 is not provided with respect to the sensors SC1, SC3, andSC4. However, a configuration in which the check patch #1 is included inthe average value processing may be employed. Alternatively, aconfiguration may be employed in which, in order to specify a checkpatch having a large color difference to the average value, the colordifference between the average value of the color values of the checkpatches #1 to #13 and the color value of each check patch is obtained,and it is determined whether or not this color difference is smallerthan or equal to a threshold value, that is, 0.8 or smaller in FIG. 11.Note that the threshold value can be determined based on a permeablecolor difference index of grade AA or grade AAA provided by Japan ColorResearch Institute, for example. Note that an unevenness ofapproximately 1.0 exists in color difference within a sheet.Accordingly, in the case of calculating a color difference using thecolorimetric measurement values themselves rather than using the averagevalue of the color values of the plurality of check patches as for theintra-sheet difference and the inter-sheet difference, a configurationin which a threshold value of the color difference calculated using thecolorimetric measurement values is set to be larger than the thresholdvalue of the color difference using the average value may be employed,giving consideration to the unevenness within the sheet. For example, inthis example, the threshold value for determining the intra-sheetdifference and the inter-sheet difference is set to be larger than thethreshold value for determining the inter-sensor difference by 0.8.

The reason for selecting a color signal value of 0% for the checkpatches is because, even if check patches are formed using the samecolor signal value, the amount of attached toner varies slightly amongthe plurality of check patches. To achieve an accurate test, thethreshold values need to be set to small values. If the amount ofattached toner varies slightly, there is a possibility that the colordifference among the plurality of check patches exceeds the thresholdvalues, and therefore, in the following description, the check patchesare formed based on the color signal value of 0%. Note that aconfiguration in which the check patches are formed using a value largerthan 0% as the color signal value may be employed, although the accuracydecreases. Furthermore, arrangement positions of the check patches arenot limited to those shown in FIGS. 7A to 7C. For example, in order tocheck the intra-sheet difference, at least two check patches to bedetected by the same color detection sensor need only be provided withinthe same sheet. In order to check the inter-sheet difference, a checkpatch to be detected by the same color detection sensor 200 need only beprovided on at least two sheets with respect to each color detectionsensor 200. Note that the reason why the check patches #1 and #3 areformed between a main-scan trigger patch and a pre-scan trigger patch isfor causing the color detection sensor 200 to measure the check patchesin a state where the conveyance rollers 2002 and 2004 are holding thesheet. If a sheet is not held between the conveyance roller 2002 and theconveyance roller 2004, there is a possibility that the sheet will flap.For this reason, the positions of the check patches #1 and #3 aredetermined such that the check patches are formed at positions in whichthe sheet reaches a measurement position in a state where the conveyanceroller 2002 and the conveyance roller 2004 are holding the sheet. Notethat a check patch that is formed on at least two sheets and detected bythe same color detection sensor 200 may be configured to be formed atthe same position on the respective sheets. Furthermore, in order tocheck the inter-sensor difference, the check patches to be detected byeach color detection sensor need only be provided at close positions. Inthis example, each check patch is provided at the same position in thesheet conveyance direction.

Note that, in order to understand unevenness in conveyance, colorpatches, that is, any of the colorimetric measurement patches, themain-scan trigger patches, and the pre-scan trigger patches, may belocated on the front side or the rear side, or both of these sides ofthe check patches in the sheet conveyance direction. This is because, ifconveyance is performed unevenly, the color differences do not satisfythe conditions in FIG. 11 as a result of performing colorimetricmeasurement on a part of the color patches at the time of colorimetricmeasurement for the check patches, and accordingly, deterioration in thecolorimetric measurement accuracy due to unevenness in conveyance can bedetected. Note that, for example, a configuration in which two checkpatches are formed continuously may also be employed.

If a color difference that exceeds the threshold value exists, theprinter controller CPU 313 displays a message indicating that a colordifference exceeds the threshold value, on the operation panel 180. If acolor difference that exceeds the threshold value exists, it is likelythat the value of the measurement result obtained by the color detectionsensor 200 is not correct. For this reason, there is a possibility thata tone jump occurs in the ICC profile generated based on thiscolorimetric measurement result. When a color difference that exceedsthe threshold value exists, the printer controller CPU 313 does notgenerate or update the ICC profile. On the other hand, if a colordifference that exceeds the threshold value does not exist, themulti-color table generation unit 308 updates or generates the ICCprofile based on the result of colorimetric measurement performed on thecolorimetric measurement patches in this test pattern.

FIG. 8 is a flowchart of automatic hue correction according to thisembodiment. Upon the user instructing the printer controller CPU 313 toexecute automatic hue correction through the operation panel 180, theprinter controller CPU 313 starts the processing in FIG. 8. In step S10,the printer controller CPU 313 forms the test patterns shown in FIGS. 7Ato 7C on sheets. Note that, at this time, color processing of the colorprocessing unit 315 is configured not to be performed. The printercontroller CPU 313 acquires, in step S11, color values from the sensorSC1 to SC4, calculates, in step S12, respective color differencescalculated from the color values of the check patches, and determines,in step S13, whether or not the respective color differences satisfy theconditions in FIG. 11. If the color differences satisfy the conditions,in step S14, the multi-color table generation unit 308 generates orupdates the ICC profile. On the other hand, if there is a colordifference that does not satisfy the conditions in FIG. 11, the printercontroller CPU 313 displays, on the operation panel 180, a messageindicating that an error has occurred in the automatic hue correction(colorimetric measurement result), that is, the conditions are notsatisfied.

Note that, although the content displayed on the operation panel 180 maybe only about whether or not an error has occurred in the automatic huecorrection, displaying more detailed information can improve conveniencefor the user or a maintenance person. Specifically, a configuration inwhich the measurement result that exceeds the threshold value and thevalue thereof are displayed may be employed. A configuration in whichinformation regarding an item to be checked is displayed in accordancewith the measurement result that exceeds the threshold value may also beemployed. For example, a configuration may be employed in which, if anerror has occurred in the result regarding the intra-sheet difference, amessage indicating that the backup roller 2003 is to be inspected isdisplayed since it is likely that flapping has occurred. Furthermore, aconfiguration may be employed in which, if an error has occurred in theresult regarding the inter-sheet difference, the sensor corresponding tothe error is displayed, and a message indicating that the detectionposition, uncleanness, or the like of this sensor is to be checked, or amessage indicating checking of whether or not a sheet is folded isdisplayed. Furthermore, a configuration in which a message indicatingthat the backup roller 2003 of the sensor corresponding to the error isto be inspected is displayed may be employed. Also, a configuration maybe employed in which, if an error has occurred in the result regardingthe inter-sensor difference, the sensor corresponding to the error isdisplayed, and a message indicating that the detection window of thissensor, the white reference plate 250, or the like is to be checked isdisplayed. For example, guidance for prompting the user to clean thedetection window is displayed on a display unit of the operation panel180. Furthermore, a configuration may be employed in which, if an errorhas occurred in a largest color difference, the sensor corresponding tothe error is displayed, and a message indicating that the detectionposition, uncleanness, or the like of this sensor is to be checked, amessage indicating that the backup roller 2003 of the sensorcorresponding to the error is to be inspected, and the like isdisplayed.

As described above, according to this embodiment, in the automatic huecorrection, the check patches, which are white solid portions, areprovided at predetermined positions in the test patterns, and it isdetermined whether or not the colorimetric measurement accuracy has beenmaintained, based on the colorimetric measurement result regarding thecheck patches. If it is determined that the colorimetric measurementaccuracy has been maintained, the ICC profile is created based on thecolorimetric measurement result regarding the colorimetric measurementpatches. On the other hand, if it is determined that the colorimetricmeasurement accuracy has not been maintained, an error message isdisplayed on the operation panel 180, and the ICC profile is notgenerated based on this colorimetric measurement result. With thisconfiguration, the accuracy of the automatic hue correction ismaintained, and the quality of the image to be formed is maintained.

Note that although this embodiment has described the test patterns forchecking the intra-sheet difference, the inter-sheet difference, and theinter-sensor difference, the present invention is not limited thereto.For example, if a plurality of color detection sensors are used but theentire test pattern can be printed on one sheet, a configuration inwhich only the intra-sheet difference and the inter-sensor differenceare checked may be employed. If the test patterns expand over aplurality of sheets but only one color detection sensor is used, aconfiguration in which only the intra-sheet difference and theinter-sheet difference are checked may be employed. That is to say, aconfiguration may be employed in which any of the intra-sheetdifference, the inter-sheet difference, the inter-sensor difference, andthe largest color difference, or any combination thereof is checked.

Second Embodiment

With the test patterns in FIGS. 7A to 7C, the main scan is started uponthe main-scan trigger patches being detected. In this case, the checkpatches #1 and #6 to #13 that are arranged near the main-scan triggerpatches are not readily affected by unevenness in the sheet conveyancespeed. On the other hand, the check patches #2 to #5 that are arrangedat positions distant from the main-scan trigger patches are readilyaffected by unevenness in the sheet conveyance speed. For example, asshown in FIG. 9A, at the time of the main scan, a colorimetricmeasurement patch located in the front of the check patch #2 is set as acolorimetric measurement patch C#1, and a colorimetric measurement patchlocated in the rear of the check patch #2 is set as a colorimetricmeasurement patch C#2. If the conveyance speed is high, there is apossibility that a part of the colorimetric measurement patch C#2 issubjected to colorimetric measurement at the timing of colorimetricmeasurement for the check patch #2. On the contrary, if the conveyancespeed is low, there is a possibility that a part of the colorimetricmeasurement patch C#1 is subjected to colorimetric measurement at thetiming of colorimetric measurement for the check patch #2.

For this reason, according to this embodiment, the color of thecolorimetric measurement patches in the front and rear of the checkpatch is set to a color at a distant position in the color space. Forexample, as shown in FIG. 9B, the colorimetric measurement patch C#1 isformed of 40% cyan and yellow toner and 0% other color toners. Thiscolor is pale green, and is at a position in the second quadrant on theplane coordinates with complementary color axes a* and b* shown in FIG.9C. On the other hand, the colorimetric measurement patch C#2 is formedof 40% magenta and yellow toner and 0% other color toners. This color ispale red, and is located at a position in the first quadrant on theplane coordinates with the complementary color axes a* and b* shown inFIG. 9C. Note that the check patch #2 is solid white, and is located atthe origin on the plane coordinates with the complementary color axes a*and b* shown in FIG. 9C.

For example, if the conveyance speed increases and a part of thecolorimetric measurement patch C#2 is subjected to colorimetricmeasurement at the timing of colorimetric measurement for the checkpatch #2, the color value of the check patch #2 is not at the origin butat a position in the first quadrant. On the other hand, if theconveyance speed decreases and a part of the colorimetric measurementpatch C#1 is subjected to colorimetric measurement at the timing ofcolorimetric measurement for the check patch #2, the color value of thecheck patch #2 is not at the origin but at a position in the secondquadrant. Accordingly, if the colorimetric measurement result regardingthe check patch does not satisfy the conditions in FIG. 11, it can bedetermined whether or not a problem has occurred in the conveyancespeed, based on the quadrant to which the color value of the check patchhas moved.

Note that the color patches in the front and rear of the check patch maybe configured not to be formed using black toner. This is to distinguishwhether the reason why the conditions in FIG. 11 are not satisfied isdue to unevenness in the conveyance speed or due to flapping of thesheet. For example, if the distance between the color detection sensor200 and the sheet increases because the backup roller 2003 cannotsufficiently suppress the flapping, the brightness, that is, the valueof L* of a detected color decreases. That is to say, if the value of L*is lower than the average value, it is conceivable that flapping hasoccurred due to a problem with the backup roller 2003. However, if blacktoner is used in the color patches in the front and rear of the checkpatch, it cannot be distinguished whether flapping has occurred or thereis a problem in the conveyance speed. In the case of not using blacktoner in the color patches in the front and rear of the check patch, ifthe brightness is lower than the average value, it is likely that thesheet has flapped, and accordingly, the backup roller 2003 is subjectedto a check. On the other hand, if the color value of the check patch hasmoved to the same quadrant as that of the color patch in the front andrear of the check patch, it is likely that the conveyance speed isuneven, and accordingly, the conveyance roller is subjected to a check.Accordingly, the item to be checked can be more specifically indicatedfor the user or the maintenance person based on the measure result. FIG.12 shows an example of measure results, causes thereof, and contentdisplayed on the operation panel 180.

Other Embodiments

Embodiments of the present invention can also be realized by a computerof a system or apparatus that reads out and executes computer executableinstructions (e.g., one or more programs) recorded on a storage medium(which may also be referred to more fully as a ‘non-transitorycomputer-readable storage medium’) to perform the functions of one ormore of the above-described embodiments and/or that includes one or morecircuits (e.g., application specific integrated circuit (ASIC)) forperforming the functions of one or more of the above-describedembodiments, and by a method performed by the computer of the system orapparatus by, for example, reading out and executing the computerexecutable instructions from the storage medium to perform the functionsof one or more of the above-described embodiments and/or controlling theone or more circuits to perform the functions of one or more of theabove-described embodiments. The computer may comprise one or moreprocessors (e.g., central processing unit (CPU), micro processing unit(MPU)) and may include a network of separate computers or separateprocessors to read out and execute the computer executable instructions.The computer executable instructions may be provided to the computer,for example, from a network or the storage medium. The storage mediummay include, for example, one or more of a hard disk, a random-accessmemory (RAM), a read only memory (ROM), a storage of distributedcomputing systems, an optical disk (such as a compact disc (CD), digitalversatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, amemory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2015-158497, filed on Aug. 10, 2015, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image forming apparatus comprising: an imageformer configured to form an image on a sheet; a sheet conveyerconfigured to convey the sheet along a conveyance path; a sensorassembly configured to measure reflected light from a test sheetconveyed by the sheet conveyer, the sensor assembly including a firstsensor and a second sensor, the second sensor being provided at aposition different from a position of the first sensor in a directionperpendicular to a conveyance direction in which the sheet is conveyedby the sheet conveyer; and a controller configured to control the imageformer to form, on the sheet, a plurality of measurement images forcreating the test sheet, to control the sheet conveyer to convey thetest sheet, to control the sensor assembly to measure the reflectedlight from the test sheet, and to generate information regarding a colorof the plurality of measurement images based on a measurement resultobtained by the sensor assembly, wherein the controller controls theimage former to form a first plurality of measurement images forcreating a first test sheet, and controls the image former to form asecond plurality of measurement images for creating a second test sheet,wherein the controller detects a sensor that has output an abnormalmeasurement result, based on a measurement result regarding reflectedlight from a first predetermined region of the first test sheet obtainedby the sensor assembly, and based on a measurement result regardingreflected light from a second predetermined region of the second testsheet obtained by the sensor assembly, wherein the first predeterminedregion corresponds to a region on which the first plurality ofmeasurement images are not formed, and wherein the second predeterminedregion corresponds to a region on which the second plurality ofmeasurement images are not formed.
 2. The image forming apparatusaccording to claim 1, further comprising: an operation panel configuredto give notice of a detection result by the controller.
 3. The imageforming apparatus according to claim 1, wherein the sensor assemblyfurther includes: a first window provided between the first sensor andthe test sheet; and a second window provided between the second sensorand the test sheet.
 4. The image forming apparatus according to claim 3,further comprising: an operation panel configured to give notice ofguidance for prompting a user to clean the first window if the firstsensor is detected by the controller, and give notice of guidance forprompting the user to clean the second window if the second sensor isdetected by the controller.
 5. The image forming apparatus according toclaim 1, wherein the sensor assembly includes a first reference memberthat is measured by the first sensor, and a second reference member thatis measured by the second sensor, and wherein the controller generatesthe information based on a measurement result obtained by the sensorassembly, a measurement result of the first reference member obtained bythe first sensor, and a measurement result of the second referencemember obtained by the second sensor.
 6. The image forming apparatusaccording to claim 1, wherein a measurement position of the first sensorand a measurement position of the second sensor are arranged next toeach other at predetermined positions in the direction perpendicular tothe conveyance direction.
 7. The image forming apparatus according toclaim 1, wherein the first predetermined region of the first test sheetin the conveyance direction is the same as the second predeterminedregion of the second test sheet in the conveyance direction.